Adolescents in Nairobi's informal settlements often face nutritional deficiencies and excesses that affect their health outcomes. A mixed-methods approach combining anthropometric measurements with qualitative interviews to assess dietary changes over time. Adolescents who participated in the intervention showed a significant reduction in body mass index (BMI) by 10% compared to controls, indicating healthier weight trajectories. Dietary interventions can effectively mitigate excessive weight gain among adolescent participants in Nairobi's informal settlements. Promoting sustainable and culturally appropriate dietary education programmes should be prioritised for long-term health benefits. Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.